1. Field of the Invention
The present invention relates to a method and circuit for generating a single-sideband optical signal in optical fiber communication employing a single-sideband (SSB) optical modulation system. More particularly, the present invention relates to a method and circuit for generating a single-sideband optical signal without necessitating a 90-degree hybrid circuit.
2. Description of the Related Art
To increase the transmission capacity of an optical-fiber communication system, the use of wavelength division multiplexing (WDM) together with the realization of higher bit rates is effective. Following several WDM optical transmission systems have recently been put into practical use. According to one of these systems, an optical signal having a bit rate of 10 Gb/s per wavelength is wavelength-multiplexed at a frequency interval of 50 GHz (corresponding to a wavelength interval of 0.4 nm in a waveband of 1.55 μm), and the resultant optical signal is transmitted.
For higher density multiplexing, an optical transmitter using a single-sideband (SSB) modulation system can be used. According to this system, the spectral width of signal is narrow. The SSB modulation system is one technology generally used in the radio communication field. When the SSB modulation system is applied to optical communication, there is a lot of similarity between the configuration of the optical transmitter in optical communication and that in radio communication. Recent optical modulators use a circuit including a Mach-Zehnder interferometer, which can shift the phase up to 90 degrees due to bias adjustment.
This type of method or circuit for generating a single-sideband optical signal includes, for example, an optical signal amplifier and an optical transmission system disclosed in Japanese Unexamined Patent Publication No. S61-13231. As an integrated optical device of the amplifier, for example, an SSB modulator 110 shown in FIG. 1 is used. In the SSB modulator 110, an optical waveguide is disposed on a lithium niobate (LiNbO3) substrate to form a Mach-Zehnder interferometer. An appropriate electric field is applied to electrodes 111 to 113 on the waveguide to generate an SSB optical signal.
As shown in the diagram, continuous wave (CW) light emitted from a semiconductor laser 120 is split into two carrier waves. For one carrier wave, a 90-degree phase-shifting bias is applied to the electrode 111 to shift the phase of light serving as the carrier wave by 90 degrees (π/2). After that, a microwave signal in an intermediate frequency (IF) band is applied to the electrode 112 to modulate the phase. For the other carrier wave, the phase of the microwave signal is shifted by 90 degrees through a 90-degree phase shifter 114 functioning as a 90-degree hybrid circuit. The resultant microwave signal is then applied to the electrode 113 to modulate the phase of the other carrier wave. Combining the individual phase-modulated carrier waves can produce an SSB optical signal as an output.
As another case, an SSB modulator 210 shown in FIG. 2 is cited. This modulator was disclosed by Shimozu et al. in the 25th Optical Fiber Communication Conference (OFC 2000 conference) in March 2000. The SSB modulator 210 is formed on a lithium niobate substrate. The SSB modulator 210 includes a sideband generating section 211 and a sideband suppressing section 212. The sideband generating section 211 is constructed in such a manner that Mach-Zehnder interferometers 214 and 215 are integrated in both arms of a Mach-Zehnder interferometer 213.
The Mach-Zehnder interferometer 214 is driven by a single frequency signal of 10 GHz and the inverted signal thereof. The other Mach-Zehnder interferometer 215 is driven by a signal having a 90-degree phase difference with respect to the single frequency signal of 10 GHz and the inverted signal thereof.
In other words, the Mach-Zehnder interferometer 213 drives the two Mach-Zehnder interferometers 214 and 215 on both arms with the above-mentioned 10-GHz single frequency signals having the appropriate phase difference, thereby generating an SSB optical signal. The Mach-Zehnder interferometer 213 performs push-pull modulation on the two Mach-Zehnder interferometers in both arms, thus performing 0-π phase modulation without causing undesirable chirping.
The SSB modulator 210 adjusts a bias applied to either port or both ports of each of the Mach-Zehnder interferometers 214 and 215, thereby causing a phase difference. The phase difference between the carrier waves of the optical signals in both arms is set to 90 degrees. That is, a phase difference can still be generated even when a dedicated electrode for 90-degree phase shifting is not disposed as described with reference to FIG. 1.
As another example, for example, a waveguide type optical modulator and an optical modulating method disclosed in Japanese Unexamined Patent Application Publication No. H11-249094 are cited (not shown as any diagram). According to this method, a carrier wave having a frequency ω0 is SSB-modulated to generate an SSB optical signal in a manner similar to the SSB modulator described with reference to FIG. 2. An optical signal having a frequency ω1 different from the above frequency is combined with the above SSB optical signal. The resultant optical signal is transmitted According to this method, the carrier wave having the frequency ω0 is converted into two carrier waves (frequencies ω1 and ω2). The optical signal having the frequency (ω1) is mixed to perform cancellation, and therefore, only the frequency (ω2) remains.
The above-mentioned conventional methods and circuits for generating a single-sideband optical signal have the following disadvantages.
First, for microwaves, an SSB optical signal cannot be generated using a wideband baseband signal, which is generally used in optical communication.
The reason is as follows. The 90-degree hybrid circuit is used for 90-degree phase shifting of microwaves. However, the 90-degree hybrid circuit does not operate over a wide frequency band from a low frequency close to DC up to a high frequency.
Second, it is difficult to realize a chirpless optical phase modulator. If such modulator can be realized, it prevents the realization of high-density multiplexing.
The reason is as follows. Non-linearity is caused due to the voltage versus optical output sinusoidal characteristics of the Mach-Zehnder interferometer. An excess high-frequency component remains in the SSB optical signal due to the non-linearity.
The above problems will now be described with reference to FIGS. 3 to 5.
Referring to FIG. 3, an SSB modulator 310 used in this case is the same type as the sideband generating section in FIG. 2. Namely, the SSB modulator 310 is constructed in such a manner that Mach-Zehnder interferometers 312 and 313 are integrated in both arms of a Mach-Zehnder interferometer 311. A 90-degree phase shifter 314 is connected in series with the Mach-Zehnder interferometer 313. The SSB modulator 310 performs combination to generate an SSB optical signal. To obtain data in this instance, an NRZ (Non Return to Zero) data signal at a bit rate of 10 Gb/s is used as a data signal to drive the SSB modulator 310.
Accordingly, the Mach-Zehnder interferometer 312 is driven by the data signal at 10 Gb/s and the inverted signal thereof, thereby generating an optical signal sa. The other Mach-Zehnder interferometer 313 is driven by a signal having a 90-degree phase difference with respect to the data signal at 10 Gb/s and the inverted signal thereof. An output of the Mach-Zehnder interferometer 313 is supplied to the 90-degree phase shifter 314 to become an SSB optical signal sb. The optical signal sa and the optical signal sb are combined into an SSB optical signal sc.
Subsequently, the states of the individual optical signals sa, sb, and sc shown in phasor diagrams will now be described with reference to FIGS. 4A to 4C. The phasor diagrams show the amplitude and the phase of light at each time by a vector. The amplitude indicates the distance from origin and the phase denotes the rotation angle from the X axis. In the case of the modulated optical signal, since the position of the vector moves with the passage of time, as shown in FIGS. 4A to 4C, the phasor diagram is useful to intuitively understand the motion or path of the modulated optical signal.
The Mach-Zehnder interferometer 312 is driven in a push-pull manner by a data signal having the same amplitude as that of a half-wavelength voltage (V π) and the inverted data signal thereof. In this instance, both a mark (1) and a space (0) of the data signal are allowed to match the peak of the output characteristics of the Mach-Zehnder interferometer 312, so that a 0-π phase-modulated optical signal can be obtained, as shown in the figure. Similarly, the other Mach-Zehnder interferometer 313 is driven in push-pull manner by a 90-degree phase-shifted delayed data signal and a 90-degree phase-shifted delayed inverted data signal, thereby obtaining phase-modulated optical signal. The above two phase-modulated optical signals are combined so as to produce a 90-degree optical phase difference, so that the SSB optical signal sc can be obtained. The following is clear from the phasor diagrams. When the vector travels between the mark (1) and the space (0), the vector moves due to phase rotation without returning to origin. The above characteristics are peculiar to the SSB optical signal.
FIGS. 5A and 5B show the optical spectrum and the homodyne detection reception waveforms of the SSB optical signal sc.
It can be understood from the optical spectrum that low frequency components are suppressed compared with the carrier wave component and the carrier frequency. The SSB optical signal is combined with carrier wave light having the same frequency as that of the carrier wave. Then, when the resultant light is received by a photoreceiver, the data signal can be demodulated by homodyne detection. As mentioned above, in the case where the carrier wave light is generated in a receiver and is then combined with the SSB optical signal and the resultant light is received, such a configuration is called an optical homodyne receiver. The carrier wave light can also be combined with the SSB optical signal in a transmitter and the resultant light can be transmitted therefrom. In this case, a standard direct detection receiver can be used as a receiver. However, in this case, it should be noted that homodyne detection reception is also performed. The frequency of the carrier wave light to be combined for the optical homodyne detection is equivalent to the carrier frequency of the SSB optical signal.
According to the foregoing methods, the SSB optical signal can be produced as mentioned above but undesirable components remain in the optical spectrum. Since, in the phasor diagram, the trace is not circular, that is to say, the distance from origin changes, it is understood that a residual intensity-modulated component exists.